1. Technical Field of the Invention
The present technology relates to diagnostic devices for a temperature sensor provided in an automobile transmission, for example, and in particular to a diagnostic device having a simple configuration that detects with high accuracy malfunctions in which a detected value by the temperature sensor is offset.
2. Related Art
Automatic transmissions for automobiles, for example, are provided with a temperature sensor for detecting the oil temperature of automatic transmission fluid that functions as hydraulic fluid and lubricant.
Since malfunctions in the temperature sensor may cause problems with control of the automatic transmission, various methods have been proposed for detecting malfunctions.
For example, a technique disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. 2004-11869 discloses diagnoses malfunctions in an oil temperature sensor in that case where the ambient temperature of a vehicle and a detected value by the oil temperature sensor are compared and the detected value of the oil temperature is lower than the ambient temperature.
In addition, JP-A No. 2008-107089 discloses a technique to diagnose an occurrence of temperature drift for a thermocouple temperature sensor. The technique gives a weight which depends on temperature to the time of use at which the temperature drift may occur and diagnoses the occurrence of the temperature drift on the basis of a drift amount estimated from an additive value of this usage time after the weight has been added.
The diagnosis of malfunctions in temperature sensors having thermistors is comparatively easy when the output voltage is clearly outside a normal range, as with supply faults, ground faults, and wiring disconnects.
However, in the cases that, for example, the contact resistance is increased due to degradation of the connector fitting, a ground fault via contact resistance occurs due to interference with other metallic parts, which are a state similar to when resistance is applied to the sensor in series or in parallel, it is difficult to detect malfunctions despite the detected values being offset toward low temperatures or high temperatures as the actual output voltage is still within normal ranges.